Radio-frequency heating of saline water: The dipolar and nonlinear absorption

The dominant mechanism of radio-frequency (RF) heating in saline water, which is critical to biomedical and food-processing applications, has conventionally been attributed to electric dipolar polarization owing to the long wavelength of RF waves. In this work, we challenge this prevailing view by demonstrating that magnetic dipole absorption plays a decisive role, even in subwavelength saline samples, particularly under high salinity or elevated temperature conditions. By performing a Taylor expansion of the Mie coefficients, we derive analytical criteria for predicting the dominance of magnetic over electric dipole absorption. Furthermore, we reveal a strongly temperature-dependent nonlinear heating effect arising from the interplay between rising temperature and enhanced magnetic absorption. Our simulations reveal that incorporating this coupled nonlinearity results in predictions of temperature rise that diverge by up to 150% from those of conventional linear models. These results could provide intriguing insights into the complex thermal behaviors of biological and colloidal systems under RF exposure.